Tool bars were originally developed to provide a means for drawing several agricultural implements behind a tractor in unison in order to plant or cultivate several furrows at the same time. Such a toolbar, generally fabricated from an elongate steel beam of square, rectangular, or diamond cross-section, extends transversely behind the tractor where it is secured to the three-point tractor hitch. The implements are generally bolted to the toolbar and may be exchanged for other equipment when necessary.
As larger and more powerful tractors became available, the toolbars grew in length to accommodate as many as 24 furrows or rows of crop. As the length of the toolbars increased, transportation between fields and along roads and highways became a problem and it became necessary to develop a method of folding the toolbar to a more manageable width behind the tractor. Outer folding wings, pivotally attached to each end of a central section, were developed which could be folded upwardly to a vertical position over the central section to provide a narrower configuration during transportation. The greatest difficulty with this system is that, when the wing sections are folded, the implements attached thereto are rotated into a horizontal position. Not only does this place excessive side loading on the equipment, but when seeders or chemical applicators are being used, the contents of the hoppers or storage tanks will spill unless they are first emptied.
To overcome this problem, stack-folding toolbars were developed which utilize a pair of parallel arms extending between the central section and each wing section to form a parallelogram linkage. This linkage allows the wing sections to maintain a substantially horizontal orientation as they are hydraulically lifted upward and inward over the central section, thus overcoming the problems associated with the equipment rotating onto its side. While a significant improvement, these stack-folding toolbars are less than satisfactory in that when the wing sections are in the lowered, working position extending outward from the central section, the parallel arms allow for no vertical flexing of the wing sections. On terraced or hilly, rolling ground, this lack of vertical movement of the wing sections can cause the implements attached thereto to be forced out of their proper relationship with the soil. If the terrain drops off under one such wing section, planter gauge wheels can be lifted above the ground and seed delivery systems can be interrupted. If the terrain rises beneath such a wing section, inward binding can occur on the implements and on the wing section itself.